Core Lifter Assembly

ABSTRACT

A core lifter assembly includes a combination of a core lifter ring and a core lifter case. The ring has an axially extending portion of an outer generally frusto-conical configuration having an included taper angle β. The included taper angle β is also the taper angle of the outer circumferential surface of the ring. The included taper angle β may in alternate embodiments be ≧6°; ≧7°; ≧8°; or ≧10°. In one embodiment an upper limit of the included taper angle β may be about 30°. Thus in one embodiment the included taper angle β may be expressed by the relationship: 30°≧β≧6°. Further, the angle β may comprise any sub range within the aforementioned range of 6-30°. However in other embodiments the taper angle may exceed 30°.

TECHNICAL FIELD

The present invention relates to a core lifter assembly of a type used in core drilling.

BACKGROUND ART

A core lifter assembly is attached to a downhole end of a core barrel which in turn is carried by a core drill. When the core drill is in operation it cuts a core sample of the ground which passes through the core lifter assembly and into the core barrel. In order to retrieve the core sample once drilling has ceased the core drill is lifted from the toe of the hole. During this process the core lifter assembly grips the core so that the lifting force on the drill is transferred onto the core, breaking it from the ground. When the core barrel is retrieved either via use of a wireline, or by withdrawal of the entire core drill, the core sample is held in the core barrel by the core lifter assembly.

The core lifter assembly comprises two main components namely a core lifter case and a core lifter ring. An outer circumferential surface of the core lifter ring and an inner circumferential surface of the core lifter case are formed with complimentary tapered surfaces allowing the core lifter ring to slide axially relative to the core lifter case. The taper on the core lifter ring forms an included angle of about 4°-5°.

The core lifter ring is not a full or complete ring but commonly a split ring having a longitudinal slot. Also the ring is made of a resilient material which biases the ring toward a maximum diameter and corresponding maximum width of the slot. The slot opens to a maximum width when the core lifter ring is in an uphole position relative to the case. This is the position of the core lifter ring when an associated core drill is drilling a core sample and the core sample is entering the core barrel. During a core breaking operation the core drill is lifted in an uphole direction. However the ring is retarded by friction against the core with the effect that the case slides uphole relative to the ring. Due to the tapered surfaces of the case and ring the ring is now compressed about an outer circumferential surface of the core sample, and the width of the slot is reduced.

The core lifter ring can clamp the core sample so tightly that it is very difficult to remove the core sample once it is retrieved to the surface. In order for a drill rig operator to release the core sample from the core lifter they may strike the protruding core sample with a block of wood. If that does not succeed the operator may resort to use of a hammer or similar instrument to strike the end of the core sample.

Striking the end of the core sample usually results in damage to the end face of a sample. This is problematic as it makes it more difficult for a geologist to rotationally align the core sample with an adjacent sample. Further, the inability to easily release the core sample from the core barrel causes frustration to the drill rig operators and presents a significant safety risk.

Throughout this specification and claims the term “downhole end” is intended to denote a toe end of a bore hole while the expression “uphole end” is intended to denote a collar end. Accordingly the downhole end of a hole may be vertically above an uphole end where for example a hole is drilled upwardly or with at least a vertical upward inclination.

SUMMARY OF THE DISCLOSURE

In a first aspect there is disclosed a core lifter for a core drill comprising:

-   -   a core lifter ring having an axially extending portion having an         outer generally frusto-conical configuration formed with an         included taper angle β≧6°.

In a second aspect there is disclosed a core lifter for a core drill comprising:

-   -   a core lifter ring having N axially extending portions each         having an outer generally frusto-conical configuration formed         with an included taper angle β≧6° wherein N is an integer ≧1.

In a third aspect there is disclosed a core lifter for a core drill comprising:

-   -   a plurality of core lifter rings each ring having N axially         extending portions each having an outer generally frusto-conical         configuration formed with an included taper angle β≧6° wherein N         is an integer ≧1.

In one embodiment the angle β≧7°.

In one embodiment the angle β≧8°.

In one embodiment the angle β≧10°.

In one embodiment the angle 30°≧β≧6°

In one embodiment one or more of the axially extending portions comprises a plurality of circumferentially alternating and generally axially extending grooves and splines formed on an inner circumferential surface or an outer circumferential surface of the one or more axially extending portions.

In one embodiment the grooves extend parallel to the central axis of the ring.

In one embodiment the grooves follow a spiral path between axially opposite ends of their corresponding axially extending portion.

In one embodiment the grooves extend to and reach a large outer diameter end of their corresponding axially extending portion.

In one embodiment the grooves extend to and reach a small outer diameter end of their corresponding axially extending portion.

In one embodiment the core lifter assembly comprises a core lifter case in which the or each core lifter ring is slidably retained.

In one embodiment of the third aspect the core lifter assembly comprises a core lifter case in which the plurality of core lifter rings are slidably retained in axially spaced relationship.

In one embodiment the core lifter case comprises a tapered section for each axially extending portion, each tapered section configured to engage a corresponding axially extending portion to cause a change in diameter of the core lifter ring associated with the axially extending portion in response to axial displacement of the associated core lifter ring relative to the tapered section.

In a fourth aspect there is disclosed a core lifter for a core drill comprising: a core lifter ring having opposite downhole and uphole ends spaced by an axial distance X, an interior circumferential surface having a substantially constant major diameter for the axial distance X, and an outer circumferential surface, the interior circumferential surface and outer circumferential surface configured to form a half angle θ≧3° for at least a portion of the distance X in at least two circumferentially spaced locations wherein at the locations a wall of the ring decreases in thickness in a direction from the uphole end to the downhole end.

In a fifth aspect there is disclosed a core lifter for a core drill comprising: a core lifter ring having opposite downhole and uphole ends spaced by an axial distance X, an interior circumferential surface having a substantially constant major diameter for the axial distance X, and an outer circumferential surface, the interior circumferential surface and outer circumferential surface configured to provide the ring with a half angle θ≧3° for at least a portion of the distance X wherein a wall of the ring decreases in thickness in a direction from the uphole end to the downhole end for the corresponding portion of the distance X.

In one embodiment the angle θ≧4°.

In one embodiment the angle θ≧5°.

In one embodiment the angle θ≧6°.

In one embodiment the angle 15°≧θ≧3°.

In one embodiment the portion of the distance X is X/N where N is an integer and wherein the ring comprises N number of integrally formed portions.

In one embodiment the outer circumferential surface of the ring for one or more of the N portions has a radius that is constant in a circumferential direction for any location along the central axis and continuously decreases in an axial direction from the uphole end to the downhole end.

In one embodiment one or more of the N portions comprises a plurality of circumferentially spaced recesses on the outer circumferential surface whereby an outer circumference of the ring in one or more of the N portions comprises alternating grooves and sloped splines wherein the angle θ is a measure of an angle of slope of the spline relative to a line co-axial with a central axis of the ring.

In an embodiment wherein N≧2 the axially extending portions are arranged to taper in a common direction. In this embodiment the core lifter may further comprise, between mutually adjacent axially extending portions, one or more transition zones extending from a small diameter end of one of the axially extending portions to a larger diameter end of the other axially extending portions.

In one embodiment the one or more transition zone comprises a tapered transition zone having a substantially frusto-conical circumferential surface.

In one embodiment the one or more transition zones comprise a cylindrical transition zone having a substantially cylindrical circumferential surface.

In one embodiment the one or more transition zone comprises a tapered transition zone having a substantially frusto-conical circumferential surface and a contiguous cylindrical transition zone having a substantially cylindrical circumferential surface.

In one embodiment the frusto-conical outer circumferential surface of the transition zone slopes at a half angle of 90°>α>3°.

In one embodiment α=−β/2.

In one embodiment α=β.

In one embodiment an axial length of the one or more transition zone is substantially the same as an axial length of an axially extending portion.

In an alternate embodiment where N≧2 the axially extending portions are arranged to taper in a common direction and with a small diameter end of one axially extending portion located immediately adjacent a large diameter end of another axially extending portion. In such an embodiment there may be provided a stepwise transition between mutually successive axially extending portions.

In one embodiment a wall of the core lifter ring is formed of substantially uniform thickness for the axial length of each of the grooves.

In a sixth aspect there is disclosed a method of manufacturing a core lifter ring comprising:

-   -   injection moulding a metallic material into a mould provided         with a moulding cavity having a shape and configuration such         that when the metallic material is injected into the cavity and         subsequently set, the set material forms a core lifter ring         having one or more axially extending portions each axially         extending portion having an outer generally frusto-conical         configuration formed with an included taper angle β≧6°.

In a seventh aspect there is disclosed a method of manufacturing a core lifter case comprising:

-   -   injection moulding a metallic material into a mould provided         with a moulding cavity having a shape and configuration such         that when the metallic material is injected into the cavity and         subsequently set, the set material forms a core lifter case         capable of receiving and working with a core lifter ring which         has one or more axially extending portions each axially         extending portion having an outer generally frusto-conical         configuration formed with an included taper angle β≧6°.

BRIEF DESCRIPTION OF THE DRAWINGS

Notwithstanding any other forms which may fall within the scope of the apparatus and method as set forth in the Summary, specific embodiments will now be described, by way of example only, with reference to the accompanying drawings in which:

FIG. 1 a is an isometric view of a core lifter ring used in a first embodiment of a core lifter assembly in accordance with the present disclosure;

FIG. 1 b is a side elevation of the ring shown in FIG. 1 a;

FIG. 1 c is an end view of the ring shown in FIG. 1 a;

FIG. 1 d is an opposite end view of the ring shown in FIG. 1 a;

FIG. 1 e is a view of section A-A of the ring depicted in FIG. 1 d;

FIG. 2 a is a section view of a first embodiment of the core lifter assembly with an associated core lifter ring in a retracted position;

FIG. 2 b is a section view of the core lifter assembly shown in FIG. 2 a but with the core lifter ring in a clamping position;

FIG. 3 a is an isometric view of a core lifter ring used in a second embodiment of the core lifter assembly;

FIG. 3 b is a side elevation of the ring shown in FIG. 3 a;

FIG. 3 c is an end view of the ring shown in FIG. 3 a;

FIG. 3 d is an opposite end view of the ring shown in FIG. 3 a;

FIG. 3 e is a view of section A-A of the ring depicted in FIG. 3 d;

FIG. 4 a is a section view of a second embodiment of the core lifter assembly with an associated core lifter ring in a retracted position;

FIG. 4 b is a section view of the core lifter assembly shown in FIG. 4 a but with the core lifter ring in a clamping position;

FIG. 5 a is an isometric view of a core lifter ring for a third embodiment of the core lifter assembly;

FIG. 5 b is a side view of the core lifter ring shown in FIG. 5 a;

FIG. 5 c is an end view of the core lifter ring shown in FIG. 5 a;

FIG. 5 d is a view of section A-A of the core lifter ring shown in FIG. 5 c;

FIG. 5 e is a view of section B-B of the core lifter ring shown in FIG. 5 c;

FIG. 5 f is a section view of the third embodiment of the core lifter assembly depicted with the core lifter ring in a clamping position;

FIG. 6 a is an isometric view of a core lifter ring for a fourth embodiment of the core lifter assembly;

FIG. 6 b is a side view of the core lifter ring shown in FIG. 6 a;

FIG. 6 c is an end view of the core lifter ring shown in FIG. 6 a;

FIG. 6 d is a view of section A-A of the core lifter ring shown in FIG. 6 c;

FIG. 6 e is a view of section B-B of the core lifter ring shown in FIG. 6 c;

FIG. 6 f is a section view of the fourth embodiment of the core lifter assembly depicted in the core lifter ring in a clamping position;

FIG. 7 a is an isometric view of a plurality of core lifter rings for a fifth embodiment of the core lifter assembly;

FIG. 7 b is a side view of the core lifter rings shown in FIG. 7 a;

FIG. 7 c is an end view of the core lifter rings shown in FIG. 7 a;

FIG. 7 d is a view of section A-A of the core lifter rings shown in FIG. 7 c;

FIG. 7 e is a view of section B-B of the core lifter rings shown in FIG. 7 c;

FIG. 7 f is a section view of the fifth embodiment of the core lifter assembly depicted with the plurality of core lifter rings in a clamping position;

FIG. 8 a is an isometric view of a core lifter ring for a sixth embodiment of the core lifter assembly;

FIG. 8 b is a side view of the core lifter ring shown in FIG. 8 a;

FIG. 8 c is an end view of the core lifter ring shown in FIG. 8 a;

FIG. 8 d is a view of section A-A of the core lifter ring shown in FIG. 8 c;

FIG. 8 e is a view of section B-B of the core lifter ring shown in FIG. 8 c;

FIG. 8 f is a section view of a core lifter case incorporated in the sixth embodiment of the core lifter assembly;

FIG. 9 a is a section view of the sixth embodiment of the core lifter assembly with the core lifter ring in a retracted position;

FIG. 9 b is a view of the core lifter assembly shown in FIG. 9 a but with the core lifter ring in a clamping position;

FIG. 10 a is an isometric view of a core lifter ring for a seventh embodiment of the core lifter assembly;

FIG. 10 b is an end view of the core lifter ring shown in FIG. 10 a;

FIG. 10 c is a view of section A-A of the core lifter ring shown in FIG. 10 b;

FIG. 10 d is a view of section B-B of the core lifter ring shown in FIG. 10 b;

FIG. 11 a is an isometric view of a core lifter ring for an eighth embodiment of the core lifter assembly;

FIG. 11 b is a side view of the core lifter ring shown in FIG. 11 a;

FIG. 11 c is an end view of the core lifter ring shown in FIG. 11 a;

FIG. 11 d is a view of section A-A of the core lifter ring shown in FIG. 11 c;

FIG. 11 e is a view of section B-B of the core lifter ring shown in FIG. 11 c;

FIG. 12 a is a side view of a core lifter ring for a ninth embodiment of the core lifter assembly;

FIG. 12 b is an end view of the core lifter ring shown in FIG. 12 a;

FIG. 12 c is a view of section A-A of the core lifter ring shown in FIG. 12 b;

FIG. 12 d is a view of section B-B of the core lifter ring shown in FIG. 12 b;

FIG. 12 e is a section view of a core lifter case of the ninth embodiment of the core lifter assembly;

FIG. 12 f is a section view of the ninth embodiment of the core lifter assembly with its associated core lifter ring in the retracted position;

FIG. 12 g is a section view of the core lifter assembly shown in FIG. 12 f but the core lifter ring in the clamping position;

FIG. 12 h is an isometric view of the core lifter ring shown in FIG. 12 a;

FIG. 12 i illustrates the first variation of the core lifter ring shown in FIG. 12 a;

FIG. 12 j illustrates a variation to the core lifter ring shown in FIG. 12 i;

FIG. 12 k illustrates a further variation of the core lifter ring shown in FIG. 12 a;

FIG. 13 a is a side view of a core lifter ring for a tenth embodiment of the core lifter assembly;

FIG. 13 b is an end view of the core lifter ring shown in FIG. 13 a;

FIG. 13 c is a view of section A-A of the core lifter ring shown in FIG. 13 b;

FIG. 13 d is a section view of a core lifter case incorporated in the tenth embodiment of the core lifter assembly;

FIG. 13 e is a section view of a tenth embodiment of the core lifter assembly comprising the core lifter ring of FIG. 13 a and the core lifter case of FIG. 13 d, and showing the core lifter ring in a clamping position;

FIG. 14 a is an isometric view of a core lifter ring for an eleventh embodiment of the core lifter assembly;

FIG. 14 b is a side view of the core lifter ring shown in FIG. 14 a;

FIG. 14 c is an end view of the core lifter ring shown in FIG. 14 a;

FIG. 14 d is a view of section A-A of the core lifter ring shown in FIG. 14 c;

FIG. 14 e is a view of section B-B of the core lifter ring shown in FIG. 14 c;

FIG. 14 f is a view of detail C shown in FIG. 14 e;

FIG. 15 a is an isometric representation of the core lifter ring for a twelfth embodiment of the core lifter assembly; and

FIG. 15 b is an isometric view of a core lifter ring of a thirteenth embodiment of the core lifter assembly.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 a-1 e illustrate a core lifter ring 10 a (hereinafter referred to in general as “ring 10 a”) for a first embodiment of a core lifter assembly 12 a depicted in FIGS. 2 a and 2 b. The core lifter assembly 12 a comprises a combination of the ring 10 a and a core lifter case 14 a (hereinafter referred to in general as “case 14 a”). The ring 10 a has an axially extending portion 16 a of an outer generally frusto-conical configuration having an included taper angle β. The taper angle β is also the taper angle of the outer circumferential surface 18 a. The included taper angle β is a measure of the included angle between two diametrically opposed axial tangents T1 and T2 of the surface 18 a. The taper of the portion 16 a and surface 18 a may also be designated by a half angle θ=β/2 where θ is the angle between one of the tangents T1 or T2 and a line L drawn parallel to a central axis 20 a and extending from a maximum diameter end 22 a of the portion 16 a. Reference number 23 a designates an opposite minimum diameter end of the ring 10 a and portion 16 a.

The included taper angle β may in alternate embodiments be ≧6°; ≧7°; ≧8°; or ≧10°. In one embodiment an upper limit of the included taper angle β may be about 30°. Thus in one embodiment the included taper angle β may be expressed by the relationship: 30°≧β≧6°. Further, the angle β may comprise any sub range within the aforementioned range of 6-30°. However in other embodiments the taper angle may exceed 30°.

As shown most clearly in FIGS. 1 a, 1 c and 1 d, the ring 10 a is in the form of a split ring and provided with an axially extending slot 24 a. Also the ring 10 a is made from a resilient material such as but not limited to spring steel. As a consequence ring 10 a can be radially compressed and expanded. However the ring 10 a is configured so that in the absence of radially applied (i.e. hoop) compression it assumes its relaxed working diameter configuration. This may be greater than the maximum inner diameter of the portion of the case 14 a in which the ring 10 a resides. In this event there will be some pressure applied by the ring 12 a to the inside of the case 14 a. Conversely it may be less than the maximum inner diameter of the portion of the case 14 a.

In the present embodiment the outer circumferential surface 18 a of ring 10 a is a continuous smooth surface which progressively reduces in outer diameter in a direction from the large diameter end 22 a to small diameter end 23 a. However an interior circumferential surface 26 a of the ring 10 a is provided with alternating axially extending grooves 28 a and splines 30 a. The grooves 28 a are formed to a depth and of a shape so that a wall thickness W (shown in FIG. 1 e) of the ring 10 a along each groove 28 a is constant. Conversely, the width of the ring 10 a in an axial section through a spline 30 a is tapered at the half angle θ as shown on the left hand side of FIG. 1 e.

The free faces 32 a of the splines 30 a are radiused and lie on a common cylinder of the same radius. The actual radius of the circle will change depending on the relative juxtaposition of the ring 10 a in the case 14 a. Also although not shown in this embodiment the free faces 32 a may be provided with texturing such as circumferential ribs, grooves, dimples or grit to assist in gripping a core passing through the ring 10 a.

Referring to FIGS. 2 a and 2 b the case 14 a has an uphole end 34 a provided with an internal screw thread 36 a to enable coupling to an inner core barrel. Downhole of the thread 36 a there is provided an internal shoulder 38 a which acts as a stop to limit sliding motion of the ring 10 a in an uphole direction relative to the case 14 a. Downhole of the shoulder 38 a there is provided a tapered surface portion 40 a. The surface portion 40 a has a taper substantially complimentary to the taper of the portion 16 a. The tapered portions 40 a and 16 a cooperate with each other to effect a change in the internal diameter of the ring 10 a as the ring 10 a slides axially relative to the case 14 a.

When the ring 10 a is in the retracted position shown in FIG. 2 a it may abut or lie a relatively small distance from the shoulder 38 a. In this juxtaposition the outer surface 18 a is expanded to a maximum diameter possible as limited by the inner diameter of the contacting section of the tapered portion 40 a. Consequently the internal diameter of the ring 10 a is also at its maximum diameter under the constraint of the contacting tapered portion.

During a core breaking action when a core drill, housing the core lifter assembly 12 a, is lifted from the toe of the hole, in relative terms the ring 10 a slides axially in a downhole direction in relation to the case 14 a to a clamping position shown in FIG. 2 b. Due to the cooperation of the outer surface 18 a with the tapered portion 40 a, this results in a reduction of the internal diameter of the ring 10 a effectively clamping the ring 10 a about an outer circumferential surface of a core (not shown).

Due to the increased internal taper angle β once the core barrel with the associated core lifter assembly 12 a is retrieved less force is required to revert the ring 10 a to the retracted position to release the captured core than for a comparable prior art core lifter assembly.

FIGS. 3 a-3 e depict a lifter ring 10 b incorporated in a second embodiment of a core lifter assembly. The features of the ring 10 b which are identical in function to those of the ring 10 a are denoted with the same reference numbers but with the suffix “a” replaced with the suffix “b”. The substantive difference between the rings 10 a and 10 b is the provision of two contiguous axially extending portions 16 b. Portions 16 b are of the same configuration as each other and of the same general configuration of the portion 16 a. In particular each portion 16 b has an included taper angle β and half angle θ as described hereinabove in relation to the ring 10 a, (i.e. 30°≧β≧6° and θ=β/2). The axially extending portions 16 b are arranged to taper in a common direction and with a small diameter end of one axially extending portion located adjacent a large diameter end of the other axially extending portion. In this embodiment there is a stepwise transition between the mutually successive axially extending portions.

The ring 10 b is also provided with a longitudinal slot 24 b and each portion 16 b has a smooth continuous outer surface 18 b. Grooves 28 b and splines 30 b are formed on the inner surface 26 b of the ring 10 b. Each of the grooves 28 b has a generally concave channel surface 33 b of constant radius in any transverse plane along its axial length. As a consequence, the thickness of a wall of ring 10 b in axial section along a groove 28 b is of a form of two contiguous wedges W1 and W2 shown on the right hand side of FIG. 3 e. The splines 30 b are arranged so that axial tangents to their respective free faces 32 b lie parallel to a central axis of the ring 10 b. Thus in combination the surfaces of the splines 30 b lie substantially on a common cylinder.

A core lifter assembly 12 b which utilises the ring 10 b comprises a core lifter case 14 b shown in FIGS. 4 a and 4 b. The case 14 b has an uphole end 34 b provided with an internal screw thread 36 b to enable coupling to an inner core barrel. Downhole of the thread 36 b there is provided an internal shoulder 38 b which acts as a stop to limit sliding motion of the ring 10 b in an uphole direction relative to the case 14 b. Downhole of the shoulder 38 b there is provided two tapered surface portions 40 b and 40 b. The surfaces 40 b each have a taper substantially complimentary to the taper of the portions 16 b. The tapered portions 40 b cooperate with the portions 16 b to effect a change in the internal diameter of the ring 10 b as the ring 10 b slides axially relative to the case 14 b.

As in the first embodiment, when the ring 10 b is in the retracted position shown in FIG. 4 a it may abut or lie a relatively small distance from the shoulder 38 b and the outer surfaces 18 b have expanded to the maximum diameter possible in relation to the corresponding tapered portions 40 b. In this position the internal diameter of the ring 10 b is at a maximum within the constraints of the case 14 b.

During a core breaking action when a core drill housing the core lifter assembly 12 b is lifted from the toe of the borehole, in relative terms, the ring 10 b slides axially in a downhole direction relative to the case 14 b to a clamping position shown in FIG. 4 b. Due to the cooperation of the outer surfaces 18 b with the tapered portions 40 b, this motion results in a reduction of the internal diameter of the ring 10 b effectively clamping the ring about an outer circumferential surface of a core.

It will be appreciated that as the angle β in the present embodiments is greater than that of the prior art, there is a quicker reduction in the wall thickness of the ring 10 a along its axial length. Therefore in order to provide sufficient contact area between the interior surface 26 a of the ring 10 a and a core sample, and between the external tapered surface 18 a of the ring 10 a and the internal tapered surface 40 a of the case 14 a, to provide appropriate compressive force to grip the core sample, the ring 10 a and case 14 a must be formed of a greater wall thickness than a comparable prior art ring. This will lead to a reduced diameter core sample. This however can be avoided by virtue of the provision of two portions 16 b in the ring 10 b in accordance with the second embodiment. In this embodiment as each portion 16 b is of a shorter axial length there is less reduction in the wall thickness for the portion 16 b than the portion 16 a. But to increase the area of the inner surface 26 b to provide the appropriate clamping force on a core sample (i.e. on a par to that of a prior art ring), two portions 16 b are provided. Thus substantially the same gripping force can be achieved as the prior art but the embodiments of the ring 10 are easier to release. As explained further below, a similar effect to the use of a ring 10 with multiple sections 16 is to provide multiple single portion rings 10 each having an included taper angle β as herein before described but of a reduced axial length to the comparable single portion ring 10 a shown in FIGS. 1 a-1 e.

FIGS. 5 a-5 f depict a further embodiment of a core lifter assembly 12 c which comprises a ring 10 c and case 14 c. The ring 10 c has a single axially extending portion 16 c with an outer generally frusto-conical configuration. The difference between the rings 10 a and 10 c resides in the location of the grooves 28 c and splines 30 c. In the ring 10 c, the grooves 28 c and splines 30 c are formed on an outer circumferential surface 18 a of the ring 10 c. The surfaces 32 c of the splines together create the outer generally frusto-conical configuration of the ring 10 c. The portion 16 c has an included taper angle β measured along the splines 30 c on the outer surface 18 c. Further, as shown in FIGS. 5 d and 5 e the interior surface 26 c of the ring 10 c is textured. The texture may be formed of ribs 44 c such as a shallow helical thread like structure on the surface 26 c.

The ring 10 c operates in an identical manner to the ring 10 a, and the core lifter case 14 c can be identical to the core lifter case 14 a. Operation of the core lifter assembly 12 c is also in essence identical to that of the assembly 12 a. In particular 12 c, as the ring 10 c moves from its retracted position to its clamping position the internal diameter of the surface 26 c reduces to effectively clamp onto an outer circumferential surface of a cut core. Release of the ring 10 c upon retrieval of an associated core tube is rendered easier in comparison with the prior art by the provision of the greater taper angle β.

Due to the provision of the grooves 28 c and splines 30 c on an outer circumferential surface 18 c of the ring 10 c naturally the outer circumferential surface is no longer smoothly continuous as in the ring 10 a. Conversely however the interior surface 26 c of ring 10 c is substantially continuous (save for the texturing provided by ribs 44 c). The tapering of the outer circumferential surface of ring 10 c is provided by a reduction in a radial section width of the splines 30 c in a direction from the maximum diameter end 22 c to the minimum diameter end 23 c of the ring 10 c.

Also as a result of the alternating grooves 28 c and splines 30 c the description of the outer surface 18 c as being “substantially frusto-conical” is intended to refer to the general outer shape and configuration of an envelope encompassing the portion 16 c.

FIGS. 6 a-6 f depict a further embodiment of a core lifter assembly 12 d having a ring 10 d and case 14 d. The ring 10 d in this embodiment comprises two axially extending portions 16 d each having an outer circumferential surface of substantially frusto-conical shape or configuration. The substantive difference between the ring 10 d and the ring 10 b is that the grooves 28 d and splines 30 d in the ring 10 d are formed on an outer circumferential surface. In comparison in the ring 10 b, corresponding grooves 28 b and splines 30 b are formed on the inner circumferential surface 26 b. A further difference is that in the ring 10 d the grooves 28 d in the successive portions 16 d are arranged in axial alignment and formed with respective depths so that in an axial cross section through respective channel surfaces 33 d of the grooves 28 d the ring 10 d has a substantially constant wall thickness. This is shown most clearly with reference to FIGS. 6 c and 6 d which show the ring 10 d having a substantially constant wall thickness W along the section AA. This differs from the corresponding cross section in the ring 10 b shown in FIGS. 3 d and 3 e where the wall of the ring is in the form of two contiguous wedges W1 and W2. This difference arises because in the ring 10 b the surfaces of the grooves 28 b are not parallel to the opposing outer tapered portions of faces 18 b whereas in the ring 10 d the surfaces of the grooves 28 d are parallel to the radially aligned portions on the inner surface 26 d. A further difference is that in the ring 10 b the grooves 28 b are continuous for the whole axial length of the ring 10 b. In contrast as clearly shown in FIGS. 6 a and 6 b, in the ring 10 d the grooves 28 d do not extend for the entire axial length of their corresponding portion 16 d. This enables the portion 16 d to be made of a relatively thin wall thickness similar to the wall thickness of conventional core lifter ring however due to the provision of multiple portions 16 d, the total area of inner surface 26 d which grips a core sample is on par with that of a conventional equivalent core lifter ring. Accordingly this embodiment enables easier release of the core lifter ring 10 d due to the increased included taper angle β but provides the same or greater gripping force in comparison with a prior art ring of the same axial length.

The core lifter case 14 d is identical to the case 14 b. Thus the interaction between the ring 10 d and case 14 d and the consequential operation of the assembly 12 d is in substance the same as that described herein above in relation to the assembly 12 b.

FIGS. 7 a-7 f depict a further embodiment of the core lifter assembly 12 e. This embodiment has two separate but identical rings 10 eu and 10 ed (referred to in general as “rings 10 e”) each having a single axial extending portion 16 e. Moreover this embodiment may be seen as a variation of the embodiment 12 d where the ring 10 d is cut in a radial plane half way along its the axial length to form the two separate rings 10 e. Thus while the ring 10 d comprises a single ring with two contiguous axially extending portions 16 d, the core lifter assembly 12 e comprises two separate rings 10 e each formed with a single axially extending portion 16 e.

With specific reference to FIG. 7 f, it can be seen that in the core lifter assembly 12 e, the two separate rings 10 e are able to move independently of each other within the case 14 e. The core lifter case 14 e is formed with a circumferential shoulder 38 e to limit the sliding of the uphole ring 10 eu in an uphole direction relative to the case 14 e. Upward motion of the down hole ring 10 ed is limited by a shoulder or relatively steep outwardly tapered transition surface 48 e which extends from an upper most of the tapered surfaces 40 e to the adjacent lower tapered surface 40 e. The combination of the two separate rings 10 e provide the same gripping force on a core as the ring 10 d because the combined area of the inner circumferential surfaces 26 e is in substance the same as the area of inner circumferential surface 26 d.

FIGS. 8 a-9 b depict a further embodiment of a core lifter assembly 12 f comprising a core lifter ring 10 f and core lifter case 14 f. The ring 10 f comprises three axially extending portions 16 f each of an outer generally frusto-conical configuration with corresponding substantially frusto-conical outer surfaces 18 f all tapering in the same direction. Each of the portions 16 f is substantially the same in shape and configuration to the portions 16 d and 16 e of the rings 10 d and 10 e respectively. Each of the portions 16 f is formed with an included taper angle 6°≧β≧30°, and a half angle η=β/2. As with the rings 10 d and 10 e, the recesses 28 f and splines 30 f are formed alternately circumferentially about the corresponding outer circumferential surface 18 f of the portion 16 f. A longitudinal split 24 f is formed in the ring enabling it to compress and expand in diameter as the ring 10 f moves relative to the corresponding case 14 f.

As shown in FIG. 8 d, the wall thickness of the ring 10 f along an axial section taken through the grooves 28 f is a substantially constant uniform thickness W. The axial section through the splines 30 f is depicted in FIG. 8 e and shows that each of the splines 30 f on the outer surface 18 f tapers at the half angle θ. The inner diameter of the inner circumferential surface 26 f is constant for the axial length of the ring 10 f save for the provision of ribs 44 f which provide texturing and assist in gripping a received core.

As depicted in FIGS. 8 f, 9 a and 9 b the corresponding core lifter case 14 f has the same general shape and configuration as the case 14 e but with the provision of a further inclined surface 40 f. Thus the case 14 f has three successive tapered portions 40 f one for each of the three axially extending portions 16 f of the ring 10 f. The case 14 f also comprises a shoulder 38 f providing a stop for the motion of the ring 10 f in an uphole direction relative to the case 14 f. An internal thread 36 f is formed on the case 14 f for coupling to a core barrel. FIG. 9 a shows the relative position of the ring 10 f and case 14 f when the ring 10 f is in a retracted position. This position corresponds to the position during drilling of a hole where a core sample is entering the case 14 f and corresponding core barrel. FIG. 9 b depicts the relative juxtaposition of the ring 10 f and the case 14 f during core breaking and subsequent retrieval of the core barrel. Here the ring 10 f is moved in a downhole direction relative to the case 14 f thereby resulting in a compression of the ring 10 f and thus firm clamping and gripping of a core sample.

It will be understood by those skilled in the art that the provision of the additional axially extending portion 40 f provides additional surface area to the inner circumferential surface 26 f in comparison for example to the ring 10 e. This may assist in gripping a core in the event that the ground may be fractured or there are discontinuities in the surface of the core. However at the same time because of the increased taper angle β, retraction of the ring 10 f to the position shown in 9 a to release a captured core sample requires less force and effort in comparison with the prior art.

FIGS. 10 a-10 d depict a core lifter ring 10 g for a further embodiment of the core lifter assembly. The ring 10 g comprises two contiguous axially extending portions 16 g with alternating grooves 28 g and splines 30 g extending about outer circumferential surface 18 g of the respective portions 16 g. The ring 10 g is generally similar to the ring 10 d with the exception that the grooves 28 g extend for the full length of their respective axially extending portions 16 g. In contrast it can be seen in the ring 10 d that each of the respective grooves 28 d extends from a maximum diameter end 22 d toward but stopping short of the minimum diameter end 23 d of the respective axially extending portion 16 d. To form the grooves 28 g in a manner so that they extend for the full length of the respective portions 16 g the grooves 28 g may be cut deeper relative to the inner circumferential surface or the tapered splines may be thicker. A benefit of this is that the ring 10 g can be adapted to different size core barrels to work with different diameter core samples.

A further difference between the ring 10 g and the ring 10 d is that the splines 30 g at the free small diameter end 23 g extend axially beyond the adjacent grooves 28 g. In a core lifter assembly comprising the ring 10 g, the corresponding case will be substantially identical to case 14 d.

In the embodiments described above which comprise grooves 28 and splines 30 on the outer circumferential surface the grooves and splines are shown as extending parallel to a central axis of the ring. However this is not an essential characteristic of the core lifter rings. As shown in FIGS. 11 a-11 e, a core lifter ring 10 h for a further embodiment of the core lifter assembly is formed with two axially extending portions 16 h each having alternating grooves 28 h and splines 30 h formed about respective outer circumferential surfaces 18 h. However the grooves 28 h extend in a spiral or spiroidal type path between opposite axial ends of the ring 10 h. Similarly, the splines 30 h extend in a spiral path parallel to the grooves 28 h. Apart from this difference in the configuration of the grooves and splines, the ring 10 h is the same as the ring 10 g.

In the embodiments shown in FIGS. 5 a, 6 a and 8 a it will be recognised that if the axial length of a ring 10 is X, and the number of axially extending portions 16 is N (N being an integer ≧1), then each axially extending portion 16 has an axial length of X/N. Thus for any one of these embodiments each portion 16 is of the same axial length. However other embodiments are possible where the portions 16 are of different length. In such embodiments the different length may arise due to the inclusion of a taper, radius, transition taper or deliberate design.

FIGS. 12 a-12 h illustrate a ninth embodiment of a core lifter assembly 12 i and its associated component parts namely a core lifter ring 10 i and core lifter case 14 i. In describing the core lifter assembly 12 i the same reference numbers as used in the previous embodiments are intended to denote the same or similar features. The core lifter ring 10 i comprises four axially extending portions 16 i each having an outer generally frusto-conical configuration and a corresponding substantially frusto-conical outer surface 18 i having an included taper angle β°. Each of the portions 16 i taper in the same direction. However the ring 10 i further comprises a plurality of transition zones 17 i. A respective transition zone 17 i is disposed between respective mutually adjacent portions 16 i. Moreover, each of the transition zones 17 i extends from a small diameter end 23 i of one portion 16 i to the large diameter end 22 i of the adjacent portion 16 i. Each transition zone 17 i in this embodiment is depicted as having an outer generally frusto-conical configuration and corresponding substantially frusto-conical outer circumferential surface 19 i. Further, the surfaces 19 i slope at a half angle θ>90° in an outward direction relative to a central axis of the ring 10 i. Thus, the zones 17 i and surfaces 19 i taper in an opposite or inverse direction relative to the taper of the surfaces 18 i. More specifically in the present embodiment, the half angle α=−β/2 where the − sign indicates the direction of the taper 19 i is opposite to that of the half angle θ or the included taper angle β.

The transition zones 17 i are also depicted as extending for an axial length the same as that of the portions 16 i. However in alternate embodiments this need not necessarily be the case and indeed will not be the case where the angle |α|≠|β/2|.

The ring 10 i is further provided with a single split or slot 24 i which extends for the full axial length of the ring 10 i.

A plurality of grooves 28 i and splines 30 i are formed in an axially extending direction in each of the portions 16 i and 17 i. The grooves 28 i and splines 30 i are arranged in an alternating manner circumferentially about each of the portions 16 i and 17 i. Further the grooves 28 i and splines 30 i in mutually adjacent portions 16 i and 17 i are axially aligned.

As depicted in FIG. 12 c, the thickness of the wall of ring 10 i through a section taken through the grooves 28 i is a substantially constant thickness W.

As shown in FIG. 12 d the thickness of the wall of ring 10 i taken through a section through the splines 30 i varies in thickness in accordance with the half angle of the respective portions 16 i and 17 i on which the spline 30 i resides.

The core lifter case 14 i incorporated in the assembly 12 i is provided with a plurality of alternating inclined surface portions 40 i and 41 i. The surface portions 40 i taper at substantially the same angle as the outer surface 18 i of the splines 30 i on portions 16 i. The intervening surface portions 41 i taper in an opposite direction and at substantially the same angle as the outer surface portion 19 i of the splines 30 i on portions 17 i. As with the other core lifter cases, the case 14 i comprises a shoulder 38 i which limits relative motion of the ring 10 i in an uphole direction relative to the case 14 i; and a thread 36 i to enable threaded connection to a core barrel.

FIGS. 12 f and 12 g depict the core lifter assembly 12 i with the associated ring 10 i in the retracted and clamping positions respectively. The assembly 12 i operates in substantially the same manner as described herein above in relation to the earlier embodiments. The retracted position shown in FIG. 12 f is commensurate with the drilling phase in which: an inner diameter of the ring 10 i is at a maximum with the ring 10 i abutting, or spaced a relatively small distance from the shoulder 38 i; and a core is entering the assembly 12 i. FIG. 12 g illustrates the relative motion between the ring 10 i and case 14 i during a core breaking operation where the case 14 i is lifted in the uphole direction relative to the ring 10 i. This action causes the ring 10 i to clamp around and thus grip the core sample.

FIG. 12 i illustrates a core lifter ring 10 i′ being a variation on the ring 10 i. The ring 10 i′ differs from the ring 10 i by forming the outer circumferential surface of each of the alternating portions 16 i and 17 i with a smooth continuous surface rather than with the alternating grooves 28 i and splines 30 i.

FIG. 12 j illustrates a core lifter ring 10 i″ which differs from the ring 10 i′ by replacement of the single full length split or slot 24 i with a plurality of circumferentially spaced apart blind splits or slots 24 i″. It will be noted that alternating slots 24 i″ commence from opposite axial ends of the ring 10 i″. The replacement of a single slot 24 with multiple blind slots 24″ may be incorporated in each of the previously described embodiments.

FIG. 12 k illustrates a core lifter ring 10 i′″ which differs from 10 i by including the alternating grooves 28 i and splines 30 i on the outside circumferential surface as well as grooves 29 i on the inner circumferential surface. The grooves 29 i may be adjacent to grooves 28 i or offset from them. The grooves 29 i provide localised flexibility to facilitate radial expansion and contraction of the diameter of the ring 10 i′″. They also enable the surface area in contact with the core to be varied.

FIGS. 13 a-13 c depict a core lifter ring 10 j differing from 10 i by the provision of two contiguous transition zones 17 j and 21 j of different configuration. Each zone 17 j has an outer generally frusto-conical configuration with corresponding surface 19 j tapering in an opposite direction to portion 16 j and at a steeper angle. Each zone 21 j has a substantially cylindrical outer surface with a diameter substantially the same as that of the small diameter end of adjacent portion 16 i. In this embodiment the combined axial length of a zone 17 j and a contiguous zone 21 j is substantial the same as that of a portion 16 j although this may differ in other variations.

FIG. 13 d illustrates a core lifter case 14 j for the assembly 12 j. The case 14 j has surface portions 40 j that taper at substantially the same angle as the outer surface 18 j of the portions 16 j. Intervening surface portions 41 j taper in an opposite direction and at substantially the same angle as the surface portions 19 j. As with the other core lifter cases, the case 14 j comprises a shoulder 38 j which limits relative motion of the ring 10 j in an uphole direction relative to the case 14 j; and a thread 36 j to enable threaded connection to a core barrel. The case 14 j differs from 14 i by the inclusion of a substantially cylindrical inner circumferential surface 42 j disposed between each of the intervening surface portions 41 j and the respective mutually adjacent portions 40 j.

FIG. 13 e depicts the core lifter assembly 12 j with the associated ring 10 j in the retracted position relative to the case 14 j. As in the previous embodiments, when the ring 10 j is in the retracted position shown it abuts, or is spaced a relatively small distance from the shoulder 38 j; and the ring 10 j is expanded to the maximum diameter possible in relation to the corresponding tapered portions 40 j. The cylindrical portions 21 j on the ring 10 j and the cylindrical portions 42 j on the case 14 j combine to provide gaps 44 j that can accommodate particles of grit or foreign matter that may be present between the surfaces of the transition surfaces 19 j on the ring 10 j and the intervening surface portions 41 j on the case 14 j. This minimises the risk of jamming and facilitates a maximum retraction, and clamping action of the ring 10 j. There is also a cylindrical portion 39 j adjacent to the shoulder 38 j on the case 14 j. The cylindrical portion 39 j also provides a gap 46 j which allows the ring 10 j to fully retract relative to the case 14 j even if particles of grit or foreign matter are present between the large diameter end 22 j of the ring 10 j and the shoulder 38 j on the case 14 j.

FIGS. 14 a-14 f depict a core lifter ring 10 k that may be incorporated in yet a further embodiment of the core lifter assembly. The core lifter ring 10 k is provided with two axially extending portions 16 k each having an outer generally frusto-conical configuration tapering so as to reduce in outer diameter in a downhole direction. Each portion 16 k is formed with a half angle θ in the same range as discussed and disclosed in relation to the earlier embodiments. The outer surface 18 k of each portion 16 k is substantially smooth and continuous. However on the interior surface 26 k of the ring 10 k each of the portions 16 k is formed with circumferentially spaced grooves 28 k. Splines 30 k are formed between the grooves 28 k. The splines 30 k are in effect formed by default by machining or otherwise forming the grooves 28 k in the interior surface 26 k.

In this embodiment edges 50 delineating the grooves 28 k from the splines 30 k are not straight and are not parallel with each other. This is to be contrast for example with the grooves 28 a and splines 30 a in the embodiment shown in FIG. 1 a. From FIG. 14 f it will also be seen that the width W of the wall of the core lifter ring 10 k in an axial section taken through the grooves 28 k is substantially constant.

FIGS. 15 a and 15 b depict core lifter rings 10 l and 10 m respectively for yet further alternate embodiments of the core lifter assembly. Core lifter ring 10 l comprises three axial portions 16 l formed contiguously with each other. Each portion 16 l has an outer generally frusto-conical configuration and a continuous smooth outer surface 18 l. Each portion 16 l is formed with a taper having a half angle θ in the same range as disclosed herein before in relation to the earlier embodiments. An interior surface 26 l may be provided with texturing to enhance friction and grip on a received core. The texture may be provided for example by way of circumferential grooves, helical grooves, grit, gnurling, dimples or the like. Also provided in the surface 26 l is a plurality of longitudinally extending and circumferentially spaced apart grooves or recesses 28 l. The recesses 28 l are of a different configuration to those depicted in the earlier embodiments. Here, the recesses 28 l extend only for an axial length bound by the corresponding portion 16 l to which they relate. Further the recesses 28 l have opposite axial ends that terminate in board of the axial ends of the their corresponding portions 16 l and are axially spaced from grooves 28 l of adjacent portions 16 l. The grooves 28 l provide a plurality of edges that may assist in the process of gripping a core and also provide enhanced flexibility for the radial expansion or contraction of the ring 10 l. Further by forming the grooves 28 l in a manner so that they increase in depth in the direction from the small diameter end 22 l to the large diameter end 23 l the ring 10 l has a greater uniformity in wall thickness which may be beneficial when the ring 10 l is made using a moulding process.

The ring 10 l may be made from a moulding process such as metal injection moulding or other techniques using powder metallurgy or amorphous metal alloys. The use of for example metal injection moulding enables high speed mass production with dimensional stability; and the forming of shapes, features and textures that are difficult to produce with conventional machining.

The metal injection moulding process for the manufacture of the ring 10 l initially requires the construction of a mould having an interior configuration complimentary to the exterior shape and configuration of the ring 10 l. Fine metal powders (typically less than 20 microns) are combined with a binder into a feedstock that is granulated and fed to a conventional injection moulding machine. The machine then injection moulds the molten feedstock into the mould. The process is similar to that of conventional plastic injection moulding and high pressure die casting.

The ring 10 l may also be made by a process of injection moulding low melting point alloys or amorphous metal alloys which may not need to be powdered or combined with a binder.

The ring 10 m may also be produced by similar moulding processes used for production of the ring 10 l. The ring 10 m is formed with three axially extending portions 16 m each having an outer generally frusto-conical configuration. Moreover each portion 16 m tapers at a half angle θ in the same range as that described in relation to the earlier embodiments. The two major differences between the rings 10 m and 10 l lie in the configuration of their respective outer surfaces 18 and interior surfaces 26. In particular ring 10 m has an interior surface 26 m with substantially constant inner diameter and formed with no recesses 28. The surface 26 m may however be provided with texturing such as by way of provision of circumferential or helical grooves, grit, gnurling, dimples or the like to enhance friction between the ring 10 m and a core cut by a drill incorporating the ring 10 m.

The outer surface 18 m of each portion 16 m is formed with a pattern of triangular and diamond shaped recesses 52 and 54 respectively delineated between adjacent diamond shaped borders or ridges 56.

The interior and outer surface configurations of each of the rings 10 l and 10 m can be reversed. For example for the ring 10 l the recesses 28 l may be applied to the outer surfaces 18 l while the interior surface 26 l may be provided with texturing such as by way of provision of circumferential or helical grooves, grit, gnurling, dimples or the like. Likewise for ring 10 m the pattern of triangular and diamond shaped recesses 52 and 54 and diamond shaped borders 56 may be moved to the interior surface 26 m leaving the outer surface 18 m as smooth continuous surfaces.

While only the rings 10 l and 10 m are depicted as being manufactured by a moulding process, each of the earlier embodiments of the rings 10 a-10 k may be similarly made from a moulding process or other techniques using powder metallurgy or amorphous metal alloys. Alternately the rings 10 a-10 m may be made by the metal working techniques including stamping, pressing, machining and 3D printing. In yet a further variation, embodiments of the rings 10 a-10 m may be made from non-metallic materials including but not limited to composite materials and plastics such as polyether ether ketone (PEEK). Indeed embodiments of the core lifter case 14 a-14 j may be made via the same manufacturing techniques as described above in relation to the manufacture of the rings 10 a-10 m.

Whilst a number of specific embodiments are described, it should be appreciated that the core lifter assembly may be embodied in many other forms. For example in a variation to the embodiment of the core lifter assembly 12 e the two separate rings 10 e could be replaced with separate rings in which the alternating grooves 28 e and splines 30 e are formed on an inner circumferential surface of the ring with the outer circumferential surface being continuous as shown in rings 10 a and 10 b. Also each separate ring can be provided with two or more axially extending portions 16. For example two or more rings 10 d (FIG. 6 a) or ring 10 f (FIG. 8 a) may be used in place of the rings 10 e each of which have only a single axially extending portion. Other variations could include grooves on either the inner or outer surface or both, no grooves at all or multiple partial length slots extending from either end or both. Further, embodiments of the core lifter assembly may be formed with three individual or separate rings each having a single axially extending portion. This will be akin to a core lifter assembly similar to assembly 12 f but where the ring 10 f having three contiguous portions 16 f is split into three separate rings each having only a single portion 16 f. Further rings may be formed with more than three contiguous axially extending portions 16.

All such modifications and variations together with others that would be obvious to persons of ordinary skill in the art are deemed to be within the scope of the present invention the nature of which is to be determined from the above description and the appended claims.

It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.

In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. 

1-39. (canceled)
 40. A core lifter for a core drill comprising: at least one core lifter ring having at least one axially extending portion having an outer generally frusto-conical configuration formed with an included taper angle β≧6°.
 41. A core lifter according to claim 40 wherein each core lifter ring has N axially extending portions with each portion having an outer generally frusto-conical configuration formed with an included taper angle β≧6° wherein N is an integer ≧I.
 42. The core lifter according to claim 40 wherein the included taper angle satisfies 30°≧β≧6°.
 43. The core lifter according to claim 41 wherein the included taper angle satisfies 30°≧β≧6°.
 44. The core lifter according to claim 40 wherein one or more of the axially extending portions comprises a plurality of circumferentially alternating and generally axially extending grooves and splines formed on an inner circumferential surface or an outer circumferential surface of the one or more axially extending portions.
 45. The core lifter according to claim 44 wherein the grooves (a) extend parallel to the central axis of the ring; or (b) follow a spiral path between axially opposite ends of their corresponding axially extending portion; or (c) extend to and reach a large outer diameter end of their corresponding axially extending portion; or (d) extend to and reach a small outer diameter end of their corresponding axially extending portion.
 46. The core lifter according to claim 40 comprising a core lifter case in which the at least one core lifter ring is slidably retained.
 47. The core lifter according to claim 46 wherein the core lifter case comprises a tapered section for each axially extending portion, with each tapered section configured to engage a corresponding axially extending portion to cause a change in diameter of the core lifter ring associated with the axially extending portion in response to axial displacement of the associated core lifter ring relative to the tapered section.
 48. A core lifter for a core drill comprising a core lifter ring having opposite downhole and uphole ends spaced by an axial distance X, an interior circumferential surface having a substantially constant major diameter for the axial distance X, and an outer circumferential surface, wherein the interior circumferential surface and outer circumferential surface are configured to form a half angle θ≧3° for at least a portion of the distance X in at least two circumferentially spaced locations and wherein at the locations a wall of the ring decreases in thickness in a direction from the uphole end to the downhole end.
 49. A core lifter for a core drill comprising a core lifter ring having opposite downhole and uphole ends spaced by an axial distance X, an interior circumferential surface having a substantially constant major diameter for the axial distance X, and an outer circumferential surface, wherein the interior circumferential surface and outer circumferential surface are configured to provide the ring with a half angle θ≧3° for at least a portion of the distance X wherein a wall of the ring decreases in thickness in a direction from the uphole end to the downhole end for the corresponding portion of the distance X.
 50. The core lifter according to claim 48 wherein the half angle satisfies 15°≧θ≧3°.
 51. The core lifter according to claim 49 wherein the half angle satisfies 15°≧θ≧3°.
 52. The core lifter according to claim 48 wherein the portion of the distance X is X/N where N is an integer ≧I and wherein the ring comprises N number of integrally formed portions.
 53. The core lifter according to claim 52 wherein the outer circumferential surface of the ring for one or more of the N portions has a radius that is constant in a circumferential direction for any location along the central axis and continuously decreases in an axial direction from the uphole end to the downhole end.
 54. The core lifter according to claim 52 wherein one or more of the N portions comprises a plurality of circumferentially spaced recesses on the outer circumferential surface whereby an outer circumference of the ring in one or more of the N portions comprises alternating grooves and sloped splines wherein the angle θ is a measure of an angle of slope of the spline relative to a line co-axial with a central axis of the ring.
 55. The core lifter according to claim 41 further comprising between mutually adjacent axially extending portions one or more transition zone extending from a small diameter end of one of the axially extending portions to a larger diameter end of the other axially extending portions.
 56. The core lifter according to claim 55 wherein the one or more transition zone comprises (a) a tapered transition zone having a substantially frusto-conical circumferential surface; or (b) a cylindrical transition zone having a substantially cylindrical circumferential surface; or (c) a tapered transition zone having a substantially frusto-conical circumferential surface and a contiguous cylindrical transition zone having a substantially cylindrical circumferential surface.
 57. The core lifter according to claim 55 wherein mutually adjacent axially extending portions are arranged in a manner such that a small diameter end of one axially extending portion is located immediately adjacent a large diameter end of an adjacent axially extending portion and a stepwise transition is formed between the one axially extending portion and the adjacent axially extending portion.
 58. The core lifter according to claim 47 wherein each lifter ring comprises between mutually adjacent axially extending portions one or more transition zone extending from a small diameter end of one of the axially extending portions to a larger diameter end of the other axially extending portions and wherein the lifter case is provided with an intervening surface portion for each transition zone of an associated core lifter ring, each intervening surface portion being configured to match a corresponding transition zone of a core lifter ring associated with the transition zone to facilitate axial displacement of the associated core lifter ring.
 59. The core lifter according to claim 44 wherein a wall of the core lifter ring is formed of substantially uniform thickness for the axial length of each of the grooves. 